Germ cells populate somatic gonadal tissue, which mediates their differentiation into eggs and sperm to propagate most animal species. In the mouse a number of genes have been identified that function in early development of the gonad, providing a framework of molecular factors that act in early gonadal development. However, how these factors work together to mediate the specification and differentiation of the somatic and germ cell-derived portion of the gonad is still only very poorly understood. Many genes remain missing and the molecular mechanisms at work are still only loosely defined. Moreover, little is known in vertebrates about the maintenance, survival, and proliferation of the germ cells, or the mechanisms of their early differentiation. For example, the factors acting in the morphogenesis of the somatic and germ cell tissue into testis cords and differentiated seminiferous tubules, or the mechanisms regulating the distinctive spermatogonial cell division program and early differentiation are almost entirely unknown. Although sex determination mechanisms are not conserved between zebrafish and mammals, the molecular mechanisms driving gonadal and germ cell development downstream of the initial sex determination decision are conserved. The zebrafish provides an excellent genetic model to perform forward genetic screens to identify genes acting in vertebrate gonad development, a methodology complementary to the reverse genetic methods used in the mouse to study this process. Here, the ability to perform mutant screens in the zebrafish will be exploited to identify adult male gonadal mutants. The mutants will be characterized to determine the nature of the defect and the biological process disrupted. Lastly, the mutations will be mapped to chromosomal positions to facilitate propagating the stock and as a starting point to identify the molecular nature of the mutated gene. From these screens we expect to identify novel and important genes acting in the specification, morphogenesis, and differentiation of male gonadal tissue, the survival and proliferation of germ cells, including genes regulating spermatogonial stem cell function and the early differentiation process of cells derived from spermatogonial stem cells. These and future studies of this male sterile mutant collection will likely provide important insights, possibly targets of environmental toxicants or genes contributing to low fertility and testicular cancers in man.